Not a novel reader but from what I have read months ago in a spoiler message, he’s being secretive with his phone because he’s caught in a really bad spot with the bottom’s mother.
At the start, the mother actually paid him to befriend the bottom so he wouldn't be lonely. But now, he’s genuinely in love with him. He's hiding his phone because the mother is still contacting him, and he knows if the bottom sees those texts or calls, he'll think their entire relationship was just a paid job and that he was being lied to the whole time. He's basically trying to handle/block the mother in secret so he doesn't break the bottom's trust.
OH MY GOD THIS IS ONE HELL OF A SPOILER SJJBXJHSID
I've been reading this on and off and skipping some chapters in between Can someone tell me why the ML seems so secretive and what the hell he's hiding on his phone?